Finish hitsounding: I like to add them in parts where I can hear the cymbal crashes (like a loud "pssshh" sound) in the music. The finish hitsounding is often the easiest to do, since most of the time you just add finishes for cymbal crashes or just any loud prominent sound your hear in the music.
Clap hitsounding usually sounds good if you add claps on every 2nd and 4th beats in respect with a 4/4 measure (4 beats per measure). More often, you will hear a constant percussion (drum) sounds on every 2nd and 4th beats. One exception is music with complex beats, break rhythms, guitar riffs, and what not such as in dnb, metal, etc. Other than that, claps are considered to be relatively easy to hitsound if used carefully. Again, you can use any type of clap hitsound you like (soft-clap is often my personal preference, but I sometimes use normal-claps and drum-claps if I feel that it sounds better on some parts of the music).
Whistle hitsounding is often the hardest to hitsound and often easy to get inconsistent with them, because there are SO MANY ways to use whistles. You can use whistles for keysounds (piano for example), synths, vocals, guitar, finger snaps, and any other melodic sounds out there. You can even use whistles as the same for claps (every 2nd and 4th beats for example), usually for softer parts of the music. There are obliviously many other ways to use whistles and the possibilities are endless. For instrumental music, I usually focus on the softer parts of the music unless it contains keysounds in it.. It may helpful to look at other ranked mapsets to discover other ways you can use whistles. Remember to have fun while doing it. It's a process afterall.
Anyways, that's all I can really help you with. Hope this advice helps and good luck with hitsounding!